The present invention relates to a method of making a patterned retarder and to a patterned retarder made by such a method. Such an optical device has many applications, for instance in three-dimensional displays. The present invention also relates to an illumination source.
U.S. Pat. No. 5,327,285 discloses a process for making a patterned retarder by chemical etching or mechanical removal of a birefringent material such as polyvinyl alcohol (PVA). However, such a technique has the disadvantage that different regions of the pattern have different light absorption properties. To avoid or reduce this effect, a subsequent planarisation step may be performed but this requires an additional processing step. Further, the edge definition of the region is relatively poor. This technique cannot produce regions with different retarder orientations on a single substrate. Instead, two or more substrates must be processed and then stuck together with the correct registration.
EP 0 689 084 discloses a linearly photopolymerisable material which may be used as a patterned alignment layer for birefringent material. In order to produce a retarder having regions of different retarder orientations, two or more photolithographic steps are required in order to expose the linearly photopolymerisable material. These photolithographic steps must be correctly registered with each other, which adds to the difficulty and cost of manufacture. Further, materials of this type generally have zero or low pre-tilt and this may result in tilt disinclination walls in the birefringent material.
Patterned alignment layers for liquid crystal devices are disclosed in: xe2x80x9cFour domain TN-LCD fabricated by reverse rubbing or double evaporationxe2x80x9d, Chen et al SID95 Digest p865, xe2x80x9cTwo domain 80 deg twisted nematic LCD for grey scale applicationsxe2x80x9d, Yang, Japanese Journal of Applied Physics, vol. 31, part 2, number 11B pL1603, and xe2x80x9cA complementary TN LCD with wide viewing angle grey scalexe2x80x9d, Takatori et al, Japan Display 1992, p591. In particular, these publications disclose multi-domain LCDs for providing improved viewing performance.
xe2x80x9cPhotoalignment and patterning of LCDsxe2x80x9d, SID Information Display 12/97 describes new materials which allow processing with one mask step and controllable pre-tilt. This paper mentions that multiple rubbing techniques are unsuitable for high resolution patterns.
According to a first aspect of the invention, there is provided method of making a patterned retarder, comprising providing an alignment layer, rubbing the alignment layer in a first rubbing direction, masking with a mask at least one first region of the alignment layer to reveal at least one second region of the alignment layer, rubbing the or each second region through the mask in a second direction different from the first direction, removing the mask, disposing on the alignment layer a layer of birefringent material whose optic axis is aligned by the alignment layer, and fixing the optic axis of the birefringent layer.
The at least one first region may comprise a plurality of first regions, the at least one second region may comprise a plurality of second regions, and the first and second regions may be arranged as a regular array. The first and second regions may comprise first and second strips which alternate with each other.
The first rubbing direction may be the same as a first desired alignment direction of the or each first region and the second rubbing direction may be different from a second desired alignment direction, which is different from the first alignment direction, of the each or second region. The angle between the first and second rubbing directions may be greater than the angle between the first and second alignment directions.
The rubbing in the first rubbing direction may be lighter or harder than the rubbing in the second rubbing direction. Rubbing strength and pre-tilt may be controlled by parameters such as pile deformation (preferably set by the proximity of the substrate to the rubbing cloth), rubbing cloth rotational speed, substrate speed, number of rubs, cloth material and pile length. A lighter rub is preferably achieved by a smaller pile deformation i.e. the substrate is further from the cloth.
The birefringent material may comprise a polymerisable or cross-linkable material. The birefringent material may be polymerisable or cross-linkable by irradiation and the fixing may comprise irradiating the birefringent layer, for instance with ultraviolet radiation. The birefringent material may be polymerisable by heat or cationic polymerisation.
The birefringent layer may comprise a polymerisable liquid crystalline material. The liquid crystalline material may comprise liquid crystal monomers or oligomers or a mixture of monomers and oligomers.
The birefringent layer may comprise a diacrylatexe2x80x94containing liquid crystal material. The material may, for example, contain a mixture of mono-, di-, and tri-acrylates or epoxy resins and may contain a photoinitiator.
The birefringent layer may contain a dichroic material which may include at least one dichroic dye.
The masking may comprise photolithographically forming the mask on the alignment layer.
The masking may comprise disposing the mask as a formed mask on the alignment layer.
The alignment layer may comprise a layer which produces planar alignment of the birefringent layer, for example one of polyimide, polyamide, polyvinyl acetate and polyvinyl alcohol.
The alignment layer may be formed on a substrate, which may comprise a polariser.
The alignment layer may comprise a glass or plastics substrate.
The masking, the rubbing subsequent to the masking, and the removing may be repeated at least once for a different rubbing direction.
According to a second aspect of the invention, there is provided a method of making a patterned retarder, comprising providing an alignment layer, masking with a mask at least one first region of the alignment layer to reveal at least one second region of the alignment layer, rubbing the or each second region through the mask in a first direction, removing the mask, rubbing the alignment layer in a second direction different from the first direction, disposing on the alignment layer a layer of birefringent material whose optic axis is aligned by the alignment layer, and fixing the optic axis of the birefringent layer.
According to a third aspect of the invention, there is provided a patterned retarder made by a method according to the first or second aspect of the invention.
According to a fourth aspect of the invention, there is provided an illumination source comprising a light source, characterised by a plurality of first and second polarising beam splitters and a retarder according to the third aspect of the invention, each of the first splitters being arranged to transmit light of a first polarisation to a first region of the birefringent layer aligned by the first region of the alignment layer and to reflect light of a second polarisation orthogonal to the first polarisation to the second splitter of the pair, the second splitter of each pair being arranged to reflect the light of the second polarisation to a second region of the birefringent layer aligned by the second region of the alignment layer, and at least one of the first and second regions of the birefringent layer being arranged to change the polarisation of light from the respective splitter so that light leaving the first and second regions of the birefringent layer is in substantially the same uniform polarisation state.
It is thus possible to provide a method of making a patterned retarder using materials and photolithographic techniques in a manner which is compatible with standard liquid crystal device (LCD) fabrication. It is further possible to avoid the use of multiple photolithographic steps requiring accurate registration and alignment between each other, for instance as in EP 0 689 084. Any standard liquid crystal alignment layer may be used, including ones known to have suitable pre-tilt and voltage holding ratio properties for use as internal elements in LCD""s. Provided the pre-tilt of the alignment layer is above a predetermined level, its exact value is not critical.
It is possible to provide a flat retarder such that planarisation or additional layers are not required. Any coloration of the birefringent material, for instance caused by weak absorption in part of the visible spectrum, is uniform throughout the retarder irrespective of the alignment directions of the alignment layer. Thus, variations in coloration caused, for instance, by removal of material or subsequent planarisation as in known arrangements can be substantially avoided. It is possible to provide patterning of much finer detail than can be provided by selective removal of material through mechanical means or chemical etching. The alignment process is compatible with birefringent materials of large refractive index anisotropy, for instance of the order of 0.2, so that it is possible to make waveplates which are much thinner than those which can be produced by known techniques based on PVA. For example, a PVA half-waveplate designed for a wavelength of 500 nm would be approximately 10 to 20 xcexcm thick. Using wet chemical etching on a thick material such as this inevitably results in poor edge definition. This is a particular problem when the pitch of the pixels is of the same order as the layer thickness. Use of the present invention makes it possible to provide a waveplate which is approximately 1 to 2 xcexcm thick. Such thin devices reduce consumption of materials and provide improved edge definition by reduced mask parallax and because material does not need to be removed. Because the techniques may be provided compatible with conventional LCD fabrication processes, the devices may be provided externally or internally of LCDs.
The invention will be further described, by way of example, with reference to the accompanying drawings, in which: